the Chemistry of Semiconductor Materials

(Kimia Bahan Semikonduktor)MKS 6212/2 units

KBS – 2010 – Dr. Indriana Kartini

• Definitions and Perspectives• Electronic structure and band energy• Carrier concentration in semiconductors• Electrons flow in semiconductors • Semiconductor materials: synthesis strategy• Semiconductor materials: characterizations• Insight (individual):

– I1: Semiconductor Band Energy– I2: Interaction of Light and Matter– I3: Absorption spectra of Organic Semiconductor– I4: Absorption spectra of Inorganic Semiconductor– I5: Silicon solar cell– I6: Fullerene based solar cell

Contents

The goal of electronic materials is to generate and control the flow of an electrical current.

Electronic Materials

• Electronic materials include:1. Conductors: have low resistance which allows electrical

current flow material capable of carrying electric current, i.e. material

which has “mobile charge carriers” (e.g. electrons, ions,..) e.g. metals, liquids with ions (water,

molten ionic compounds), plasma2. Insulators: have high resistance which suppresses electrical

current flow materials with no or very few free charge carriers; e.g. quartz, most covalent and ionic solids, plastics

3. Semiconductors: can allow or suppress electrical current flow materials with conductivity between that of conductors and insulators; e.g. germanium Ge, silicon Si, GaAs, GaP, InP

Superconductors: certain materials have zero resistivity at very low temperature

Introduction_Semi_Materials-Rev7-22

• some representative resistivities (): – R = L/A, R = resistance, L = length, A = cross section area; resistivity

at 20o C resistivity in m resistance(in )(L=1m, diam

=1mm)• aluminum 2.8x10-8 3.6x10-2

• brass 8x10-8 10.1x10-2

• copper 1.7x10-8 2.2x10-2

• platinum 10x10-8 12.7x10-2

• silver 1.6x10-8 2.1x10-2

• carbon 3.5x10-5 44.5• germanium 0.45 5.7x105

• silicon 640 6x108

• porcelain 1010 - 1012 1016 - 1018

• teflon 1014 1020

• blood 1.5 1.9x106

• fat 24 3x107

Conductors

• Good conductors have low resistance so electrons flow through them with ease.

• Best element conductors include:– Copper, silver, gold, aluminum, & nickel

• Alloys are also good conductors:– Brass & steel

• Good conductors can also be liquid:– Salt water

Conductor Atomic Structure

• The atomic structure of good conductors usually includes only one electron in their outer shell. – It is called a valence

electron. – It is easily striped

from the atom, producing current flow. Copper Atom

Insulators

• Insulators have a high resistance so current does not flow in them.

• Good insulators include:– Glass, ceramic, plastics, & wood

• Most insulators are compounds of several elements.

• The atoms are tightly bound to one another so electrons are difficult to strip away for current flow.

Semiconductors

• Semiconductors are materials that essentially can be conditioned to act as good conductors, or good insulators, or any thing in between.

• Common elements such as carbon, silicon, and germanium are semiconductors.

• Silicon is the best and most widely used semiconductor.

semiconductors

AlP, AlAs

Applications:• Elemental semiconductor Ge was widely used for transistor and diodes; IR and

nuclear radiation detectors• Silicon is used for the majority rectifiers, transistors, and ICs; IR and nuclear

radiation detectors• Compound semiconductors are widely used in high-speed devices and devices

requiring the emission or absorption of light:• III-V compounds such as GaAs and GaP LEDs (light-emitting diodes)• GaAsP and InGaAsP for flexible additive in the devices• II-VI compound such as ZnS fluorescent materials for TV screen• InSb, CdSe, PbTe and HgCdTe light detectors

Semiconductor Valence Orbit

• The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.

Crystal Lattice Structure

• The unique capability of semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice.

• The atoms link together with one another sharing their outer electrons.

• These links are called covalent bonds. 2D Crystal Lattice Structure

3D Crystal Lattice Structure

Semiconductors can be Insulators

• If the material is pure semiconductor material like silicon, the crystal lattice structure forms an excellent insulator since all the atoms are bound to one another and are not free for current flow.

• Good insulating semiconductor material is referred to as intrinsic.

• Since the outer valence electrons of each atom are tightly bound together with one another, the electrons are difficult to dislodge for current flow.

• Silicon in this form is a great insulator.• Semiconductor material is often used as an

insulator.

Doping

• To make the semiconductor conduct electricity, other atoms called impurities must be added.

• “Impurities” are different elements.

• This process is called doping.

Semiconductors can be Conductors

• An impurity, or element like arsenic, has 5 valence electrons.

• Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring silicon atoms.

• The one electron left over for each arsenic atom becomes available to conduct current flow.

Resistance Effects of Doping

• If you use lots of arsenic atoms for doping, there will be lots of extra electrons so the resistance of the material will be low and current will flow freely.

• If you use only a few boron atoms, there will be fewer free electrons so the resistance will be high and less current will flow.

• By controlling the doping amount, virtually any resistance can be achieved.

Another Way to Dope

• You can also dope a semiconductor material with an atom such as boron that has only 3 valence electrons.

• The 3 electrons in the outer orbit do form covalent bonds with its neighboring semiconductor atoms as before. But one electron is missing from the bond.

• This place where a fourth electron should be is referred to as a hole.

• The hole assumes a positive charge so it can attract electrons from some other source.

• Holes become a type of current carrier like the electron to support current flow.

Types of Semiconductor Materials• The silicon doped with extra electrons is called an “N

type” semiconductor. – “N” is for negative, which is the charge of an

electron.• Silicon doped with material missing electrons that

produce locations called holes is called “P type” semiconductor. – “P” is for positive, which is the charge of a hole.

In Summary

• In its pure state, semiconductor material is an excellent insulator.

• The commonly used semiconductor material is silicon.

• Semiconductor materials can be doped with other atoms to add or subtract electrons.

• An N-type semiconductor material has extra electrons.

• A P-type semiconductor material has a shortage of electrons with vacancies called holes.

• The heavier the doping, the greater the conductivity or the lower the resistance.

• By controlling the doping of silicon the semiconductor material can be made as conductive as desired.

Important properties

• The control of semiconductor electrical and optical properties make these materials useful for electronic and photonic devices.

• The properties include, for example, electrical resistivity and optical absorption, which are related to one another by the semiconductor electronic structure.

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